Among the hallmarks of cardiac and renal diseases are abnormalities of salt and water conservation such as edema. The study of renal sodium reabsorption, a process utilizing most of the organ's energy expenditure, is hampered by the complexity of the epithelial organization of the kidney. Development of model systems such as the urinary bladder of the toad resulted in an increase in our understanding of the details of the mechanism of sodium absorption. Epithelia, such as the renal tubule and the toad bladder, have two cell borders of differing properties. Sodium transport across the toad urinary bladder is thought to occur by two discrete steps, an entry step down an electrochemical gradient and an exit step tightly linked to cellular respiration. This project proposes to study the following: 1) sodium movement across the luminal membrane and the physical and chemical factors that regulate it using a method whereby this moiety of transport is directly measured. 2) coupling of sodium exit from the epithelium to respiration will be investigated using a recently developed technique that allows continuous measurement of the total rate of respiration (CO2 producton), that fraction produced from oxidation of specifically labelled metabolic substrates as well as sodium transport; all of these three parameters will be measured simultaneously in the same tissue. This will delineate the metabolic pathways linked to transport and give quantitative estimates of the contribution of each pathway to the soduim transport mechanism. And 3) separation of the luminal and basolateral border of the cell will be attempted and transport across each of these single membranes, formed into vesicles, will be studied in situations where the media on both sides of these membranes will have known composition. Delineation of the characteristics of sodium reabsorption in this epithelium will advance our knowledge of renal function in health and in certain states, e.g., heart failure where abnormal salt conservation is a major pathophysiological manifestation.